Switching topologies are ubiquitous for high-efficiency DC/DC converter designs. This class of topologies includes both non-isolated (e.g., buck, boost, Cuk, etc.) and isolated (e.g., flyback, forward, etc.) converters. Switching converters operate by periodically drawing energy from the input voltage source, temporarily storing it in passive elements including inductors and capacitors, and transferring the energy to the output at a desired voltage or current level, which may be distinct from that of the input. Converter operation is based on the periodic switching of one or more transistors (MOSFETs, IGBTS, etc.) at a usually fixed and sometimes variable frequency.
Reducing the physical size of switching DC/DC converters is advantageous for many applications. This is true in both the low power and high power domains, for example relating to handheld devices and solar power generation topologies, respectively. One technique of reducing the size of such converters is to increase the switching frequency. This reduces the per-cycle energy storage requirement of the converter for a given power, and thereby allows reduced values and, accordingly, size of the passive energy storage elements, i.e., the inductors and capacitors.
Increasing the switching frequency, however, tends to reduce the efficiency of the converter through several mechanisms. Switching loss, which refers to the discharging of the output capacitance of the switching devices (transistors) when they are switched off, increases with frequency. Gating loss, the charging and discharging of the transistor gate capacitance (for example in MOSFET-based designs), also increases with frequency. Furthermore, the magnetic cores of inductors have increased loss at higher frequencies, resulting primarily from hysteretic and eddy-current effects. In addition, the cores saturate at high currents. While materials are being developed to enable fabrication of cores with reduced losses, it can be advantageous to use high enough frequencies, approximately 10 MHz and above, at which the inductances required are low enough such that coreless inductors without these limitations can be used.
These considerations have led to the development of quasi- and fully-resonant topologies that reduce the switching loss of high-frequency converters. The primary advantage of resonant topologies is the implementation of zero-voltage switching (ZVS) converters, in which the switching devices are switched while the voltage across them is zero and no charge is stored on the output capacitance. This significantly reduces the switching loss. Resonant switching is implemented by the use of one or more resonant tanks comprising one or more inductors and capacitors to periodically store the output charge. The switching signals are timed such that the switches are turned off when the output voltage is zero. Some fully-resonant topologies also implement resonant gating techniques in which gating loss is reduced or eliminated by periodically storing gate charge in a resonant tank.
To maintain ZVS, the interval during which quasi-resonant switches are off is fixed as half the resonant period of the quasi-resonant tank. Regulation of output power in quasi-resonant topologies is limited to variable-frequency control which requires variable oscillators and creates broadband electromagnetic interference (EMI), both undesirable characteristics. Fully resonant topologies, on the other hand, operate at a fixed frequency and fixed duty cycle. Load regulation is implemented through operating multiple converters in parallel with varying phase offsets or through using passive networks to switch between real and reactive power, see, e.g., U.S. Pat. No. 7,535,133. The former solution greatly increases the number of components in the system, reducing the advantage in miniaturization, and increasing the cost. The latter solution increases voltage stress and is limited in the range of voltage ratios and loads over which high efficiency can be maintained.